Hi, Nicole!
I've been doing 5ks and shorter races for the past 4 years, first as a speed walker (my very first race, a 2 mile walk, was one I just heard about 3 days prior, and I had no idea I'd be as fast as I was compared to others, in spite of some goofs, like taking my MP3 player and assuming the fast musíc I'd chosen would help, when it ended up being slower than my race pace, and slowing me down, especially when I tried to adjust it). That race lead to a 5k in which I won the walker's division, which started for me what has become a healthy addiction, and I've been winning or placing in races (walker division ,though sometimes getting age-group awards against runners in some races) ever since.
Here are some tips that may help you:
When is the race? Usually, 5k training programs have several weeks of prep if you aren't regularly running/walking at least close to that distance. (I had been walking 4-7 miles a day for 3-4 days a week before I entered that first race). Look up 5k training programs online (
www.runnersworld.com has some, for example) and see if they will work for you. Since you are not a regular runner (I'm not sure how much running that means you do?), don't plan to run the entire race. You will most likely need walk breaks. IMO, you should work up to doing a few run/walks at 5k distance (3.1 miles) before 'the big day,' to get used to the distance.
Some tips :
Try to get a good night's sleep the night before (not always possible, because you'll be excited and nervous: I often use "Moon Drops," a homeopathic sleep aid, the night before a race to help me sleep better.
Also, pack a race bag the night before, with the clothes you are going to wear and anything else you need (a Garmin? anti-chaffing lube? a Spi belt to hold your ID/key?)
Dress as if the temp is 10-15 degrees warmer than it actually is (depending on how much you heat up when running). Otherwise, you will get too warm after you start. If it's in the 40's or cooler, you might want to take some sweat pants and a top to wear during warm-up and to put on afterwards. One mistake I see quite a few slower runners doing is overdressing a LOT, like wearing sweat pants when it's 60 degrees. They look miserable after a very short time. (If you are driving to the race, you have the advantage of being able to take extra clothes "just in case". There are several times I've been happy that I don't clean my car out too often, as I've needed a jacket at the last minute when the weather turned.)
I've done (speedwalking, now racewalking) 2-3 5ks per month this year. In my most recent one, two weeks ago, it was about 50 degrees and windy at the start. I wore long compression tights, a long-sleeve wicking top (I do NOT wear the race tshirt, which is usually cotton, which absorbs moisture and doesn't wick. I've seen plenty of people this summer near the end of the runner's pack who were sopping wet at the end of the race because they wore the race cotton T), which I adjusted by pushing the sleeves up or bringing them down, depending on how cool/warm I felt at the time.
I also wore a buff head scarf (I first had it as a headband to cover my ears, but when the wind picked up right before the race, I changed it to a neck warmer, balakava configuration. LOVE those things! I wear them whenever the temp is below around 50, in whatever configuration works best).
I also wore a pair of light gloves (glove liners), both to keep my hands warm, and to use as nose wipers! I took a pair of sweat pants and a zipperer hoodie that I put on over my race clothes for warm-up, and to put on after the race. As always, I wore my Garmin to have my own 'official" time.
The runner's world site has a feature that you can input temperature and how you like to feel (somewhat cool, neither warm nor cool, warm), and it will give suggestions for clothing.
For food, you should not do anything 'new' on race day. Try out your food and what you are going to wear beforehand so you know what works for you. My typical pre-race fueling is a smoothie made with 1 frozen banana, a scoop of Vega pre-workout performance enhancer, a spoonful of chia seeds, and water.
Carbo loading isn't necessary, especially for that distance, but eating a healthy, higher-carb diet in general a couple of days before the race can be helpful. Also make sure you hydrate well in the days before the event.
Definitely take some time to warm up. As with everything else, you should try out your warm-up before race day. I'd recommend at least 10 minutes of an easy jog or walk, followed by some dynamic stretches. (I race for time--I like to win--and find that a longer warm-up o 15-20 minutes works better for me, especially to get my hip joints warm and flexible, but I'm 54, and definitely need the prep. I also make sure to stretch after the race, whiich many other participants dón't do).
Don't go for speed your first time out, just enjoy the experience and take in everything.
One thing I see a lot of beginners (I assume) do wrong is start out too fast. The adrenaline and excitement can get to you. Those are often the people who run a short distance, then start walking, and I pass them. ;-p As with everything else, try out your pace before race day (you will often be faster in a race than in training, because of the adrenaline rush and the thrill of competition--or maybe that's just me, LOL!--but you shouldn't be a lot faster than you usually run).
HTH!
Have fun!
luv ya Jenn! ) but I do run a couple times a week and try to hit a couple short races each year. I agree with not overthinking it too much. Just go have a good time and do what you can do. Remember- this race is all about you- have a great time while doing it! 
